The present invention relates to a prism optical system. More particularly, the present invention relates to a prism optical system for use as an ocular optical system of a visual display apparatus which comprises a two-dimensional image display device for displaying an observation image, and an ocular optical system for projecting the displayed image in the air as an enlarged image. The prism optical system has a compact size as a whole, causes a minimal reduction of the light intensity in the ocular optical system, and provides a large exit pupil diameter.
FIG. 2 is a sectional view showing the arrangement of an optical system of a conventional visual display apparatus proposed in Japanese Patent Application Unexamined Publication (KOKAI) No. 3-221934. In the figure, reference numeral 1 denotes a two-dimensional image display device, 2 a first reflecting plane, 3 a second reflecting plane, 4 a lens having positive refracting power, and 5 an observer's pupil position. In this case, display light emanating from the two-dimensional image display device 1 is successively reflected by the first reflecting plane 2 and the second reflecting plane 3 in the mentioned order and converged by the positive lens 4 to form an image displayed on the two-dimensional image display device 1 at a distance. The image is re-formed on the retinal of the observer's eyeball via the observer's pupil position 5, thereby allowing the observer to see the displayed image.
In the prior art, only the positive lens 4 has refracting power. Therefore, the principal point of the ocular optical system lies in the vicinity of the lens 4.
In general, if the observer's pupil is placed in the vicinity of the exit pupil position of an optical system, the quantity of light eclipsed is minimized. Accordingly, if the observer's pupil is placed in the vicinity of the exit pupil position 5 in FIG. 2, the focal length of the ocular optical system becomes approximately equal to the eye point distance because the principal point of the ocular optical system lies in the vicinity of the positive lens 4.
Further, an image to be viewed by the observer cannot be projected at a distance unless the two-dimensional image display device 1 is disposed in the vicinity of the front focal point of the ocular optical system. That is, the length of the optical path that extends from the two-dimensional image display device 1 to the positive lens 4 via the reflecting planes 2 and 3 also needs to be approximately equal to the focal length of the ocular optical system.
Accordingly, if no sufficiently long eye point distance is provided, it will be impossible to ensure a space sufficient for light from the two-dimensional image display device 1 to enter the positive lens 4 by being reflected by the first reflecting plane 2 and the second reflecting plane 3, giving rise to a problem that light rays are eclipsed by the plane mirrors 2 and 3 and the two-dimensional image display device 1.
However, if the focal length of the positive lens 4 is increased in order to prevent an eclipse of light rays, the eye point distance becomes excessively long, making it difficult to realize a compact ocular optical system.
Meanwhile, it is important to increase the exit pupil diameter of the ocular optical system with a view to preventing an eclipse of the observation image caused by rolling of the observer's eyeball and also to providing an image display apparatus which needs no adjustment of the interpupillary distance or the like, and which is easy to see. However, it has heretofore been difficult to increase the exit pupil diameter of the ocular optical system, that is, to reduce the F-number, in the optical system which comprises only the positive lens system 4.